问题
I am using Java & paypal express checkout & DoDirectPayment, i see there are two options either by signature given by paypal sandbox merchant account or signature given by paypal sandbox merchant account.
Do i need any certificate if i am using "signature" string ?
回答1:
At SandBox, those two Credentials (generated at SandBox primary account and test account) are same. It is for your convenience.
Do i need any certificate if i am using "signature" string? No, you do not need certificate if you use Signature. You cannot even create both in single paypal account. API Signature is new way of accessing API; even PayPal recommands using API Signature.
来源：https://stackoverflow.com/questions/5256316/java-paypal-api-call-certificate-is-requierd